The field of the disclosure described herein relates generally to monitoring gas turbine engine systems, and more specifically, to methods and systems for measuring fuel Wobbe index.
Industrial turbines are often gas-fired and commonly used to drive electrical generators in power plants. Such gas turbines are designed to burn a specific range of fuels, wherein the rate of fuel consumed may depend on the fuel's chemical composition. Nevertheless, the precise chemical composition of the fuel being burned is not immediately known due to pipeline variability and fuel-gas processing (e.g. regasification of liquid natural gas (LNG). Burning fuel of unknown composition can lead to degraded gas turbine system performance. Many problems associated with burning an unknown fuel can be abated by robust control over the gas turbine if fuel properties are measured prior to combustion, particularly if these properties are varying quickly. In addition to fuel constituent concentrations, fuel heating value and specific gravity are key fuel properties for gas turbine operation. Both are represented by the fuel Wobbe number.
The Wobbe number, or the Wobbe index, is the quotient of the volume-based lower heating value and the square root of the specific gravity of the gas. The Wobbe index is used in industry to control or maintain the amount of energy supplied to fuel gas consumers. Operation of a turbine engine at a Wobbe index outside the design range may result in undesired gas turbine combustor dynamics, such as flame flashback, low equivalence ratio (resulting in flame blowout), and high equivalence ratio or over-firing. These undesired dynamics may result in hardware distress or, possibly, a reduction in component life of the combustion system and/or a potential for power generation outage.
One of the commonly used systems for determining Wobbe index is a gas chromatograph system. The GC system includes a glass capillary system to separate fuel constituents and a thermal conductivity detector or a flame ionization detector (FID) to quantitatively identify the fuel composition. The gas chromatograph system provides measurements that are slow when compared to the time scales associated with fuel variability issues. Further, the system is relatively expensive and difficult to operate. Another commonly used system to determine fuel quality is a calorimeter, wherein the energy of fuel combustion is directly measured. Calorimeters are sensitive to both the fuel composition and the ambient temperature so that large measurement errors are common, especially when ambient temperature varies significantly.
Known methods of measuring fuel Wobbe index are costly, tedious, and slow—i.e. lacking sufficient response time for many industrial applications—particularly when robust control and fuel regulation are required. An improved system is needed for determining the fuel Wobbe index in real-time that may address one or more problems set forth above.